With a growing demand for portable electric appliances such as mobile phone, portable audiovisual device, and portable computers, demand for high-performance batteries has grown. Especially, demand for secondary battery such as nickel-cadmium battery, nickel-metal hydride battery, and lithium-ion secondary battery has increased.
Generally, those batteries are sealed ones and have the shape of cylinder or rectangular cylinder. Among them, rectangular-cylinder sealed batteries draw attention due to a space-saving advantage. Accordingly, it is assumed that there is a great demand for higher performance and reliability of rectangular-cylinder sealed batteries.
A rectangular-cylinder sealed battery is generally manufactured as follows. A bottomed, rectangular-cylinder external casing is formed by drawing a metal plate. In the external casing, a generator element composed of positive and negative electrodes are enclosed. The opening of the external casing is covered with a cover plate. When covering the opening with the cover plate, the cover plate is sealed by caulking or melting in general.
The sealing prevents electrolyte and gas from leaking out of the external casing when pressure increases inside of the external casing. The sealing quality profoundly affects the reliability and life of a battery.
Generally, mechanical caulking is often used for sealing process of battery. For rectangular-cylinder sealed batteries, however, it is difficult to adopt caulking in many cases, so that laser welding is also often used.
FIG. 25 is a conceptual diagram showing a conventional sealing technology using laser welding that has been generally adopted for sealed batteries.
FIG. 25 shows that a cover plate and an outer casing are welded together using a laser as follows. A flat cover plate 410 is fit into the opening of an external casing 400 so that the upper surface of the cover plate 410 has the same level as the top end of the rim of the external casing 400. A laser beam is intermittently aimed at the boundary between the outer edge of the cover plate 410 and the rim of the external casing 400 at a certain speed. The sealing technology using laser welding can realize the complete sealing of rectangular-cylinder sealed batteries, leading to higher reliability and longer life of rectangular-cylinder sealed batteries. As a result, the laser sealing is regarded as one core technology for higher quality of rectangular-cylinder sealed batteries.
Conventionally, the external casing and the cover plate are made of a nickel-plated steel plate or a stainless steel plate. Recently, however, an aluminum alloy plate, which is made by doping aluminum with manganese and the like, has been also used in many cases for weight reduction.
It is troublesome to use an aluminum alloy plate, however. When the external casing and the cover plate are welded using a laser, the welded part is susceptible to cracking.
Generally, cracking arises in the scanning direction of a laser beam. It is assumed that a part which has been welded using a laser beam, i.e., a welded part is pulled by heat stress that has been generated around the welded part in the course of cooling. The welded part is susceptible to cracking in the case of an aluminum alloy plate since the welded part is rapidly cooled due to the lower tensile strength and higher heat conductivity than iron and stainless.
One proposed solution to this problem is to lower the scanning speed of the laser beam since a lower scanning speed reduces the incidence of cracking. In order to curb the incidence of cracking as low as possible, the scanning speed of the laser beam is now set as relatively low for the laser welding. In terms of production efficiency, however, it is not desirable to lower the laser beam scanning speed since it takes longer time for the sealing.